The invention concerns a method and a device for finding a characteristic data group periodically inserted between other data in a digital data stream.
In the field of digital communications a known method of identifying essential elements relating to calls uses characteristic groups of digital data each conformed in a different and specifically identifiable manner. The data constituting such groups has fixed binary values and predetermined relative positions. For example, one known method uses a specific identifier group, usually called a frame alignment pattern, which is transmitted periodically to synchronize the receive part of a digital information transmission system to the send part. This technique is used when the digital data to be transmitted over a link is time-division multiplexed by the send part so that it is essential to be able to identify, at the receiving end, the various time-division channels used for transmitting the data in order to be in a position to demultiplex the data divided between successive frames and which arrives in the various time-division channels.
Various software or hardware devices have therefore been used to scan a received digital bit stream systematically, bit by bit, in order to identify a characteristic data group within it which forms a simple structured pattern, for example a small number of bits with respective fixed values one after the other.
When a simple pattern like this is introduced into a transmitted data stream, subject to given periodicity conditions, the device for finding it can be locked onto the first pattern discovered in order to attempt to find the next identical pattern, allowing for the recurrence rules chosen for this type of pattern. It remains locked on until the device in question finds an identical pattern, again allowing for the previously mentioned recurrence rules.
On the other hand, in the event of erroneous locking onto an unexpected imitation of the pattern, the search device is stumped and will be obliged sooner or later to resume the search from the beginning. This can lead to unacceptable delays when the terminals transmit large volumes of data using a lot of the transmission capacity of the transmission medium employed.
In this case the data constituting the characteristic data group is likely to be widely spaced, although the relative spacings are entirely determined.
Under these conditions the drawbacks of bit by bit searching can become unacceptable. This could arise, for example, with videophone terminals connected to an integrated services digital network (ISDN). Using its search device, when a videophone call is set up, each videophone terminal has to find a first frame alignment pattern in the data stream it receives over the digital link. Otherwise it cannot begin to communicate in the sense that it cannot display any image on its screen and only audio communication is possible.